Recording sheets for the ink jet printing process available today do not have all the required properties. Particularly there is an urgent need to improve ink absorptiveness, ink absorption rate, image quality, water fastness and light stability. A preferred embodiment of the invention relates to improved recording sheets having excellent image quality, high ink absorptiveness and high ink absorption rate. In particular ink receiving materials are needed where the images recorded thereon are resistant to rubbing on the surface and remain intact when in contact with water and should not fade when exposed to light.
Ink jet printing processes are of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. The stream is perturbed, causing it to break up into individual droplets at a fixed distance from the nozzle. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electric field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium.
In the non-continuous process, or the so called “drop-on-demand” systems, a droplet is expelled from a nozzle to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
The invention is directed towards recording sheets and coating compositions that may be used in both recording processes.
It is known that recording sheets for ink jet printing must meet a number of stringent demands. The printed image has to have the following properties:                High resolution        High colour density        Good colour reproduction        High resistance to rubbing        Good water fastness        High light stability        
The following conditions have to be met to attain these goals:                1. The ink needs to be absorbed quickly into the recording sheet.        2. The jetted ink droplets have to spread circularly on the recording sheet and have to form well defined edges.        3. Dye diffusion in the recording sheet has to be low so that the diameter of the colour points is not increased more than necessary.        4. An ink droplet is not allowed to interfere with a droplet deposited earlier nor should it blur it.        5. The recording sheet needs to have a surface that gives high colour density and brilliance.        6. The recording sheet has to show excellent physical properties before and after printing.        7. The printed recording sheet is not allowed to show changes under the influence of light.        
Recording sheets, which contain in at least one layer a porous inorganic oxide, show a particularly high ink absorption rate and good dye fixing. Pseudo-bohemite, an agglomerate of aluminium oxide/hydroxide of formula Al2O3. n H2O (n=1 to 1.5) is particularly preferred as the inorganic porous oxide. Recording sheets of this kind are described, for example, in patent applications EP 0,298,424, EP 0,407,720 and EP 0,500,021. However, these materials show a great disadvantage in that the dyes used in the ink jet printing process have bad light stability.
In order to prevent such materials containing such porous oxides from turning yellow, patent application EP 0,614,771 proposes to add organic acids with a first dissociation coefficient of at most 5 wherein these acids contain an aromatic ring or at least two carboxy groups. Particularly preferred are acids with a first dissociation coefficient between 2 and 5.
U.S. Pat. No. 4,775,594 describes the addition of non-volatile organic acids to transparent recording sheets for ink jet printing to improve the wettability of such a material by the inks.
Patent application EP 0,410,051 describes a paper with improved printability (adhesion of printing dyes) containing cinnamic acid or cinnamic acid derivatives in a surface layer.
Patent application EP 0,585,345 proposes the addition of dithiocarbamates, thiurams, thiocyanates or sterically hindered amines to recording sheets comprising porous inorganic oxides to improve light stability.
All these proposed methods, however, only give a marginal improvement of light stability of images printed on receiving sheets comprising porous inorganic oxides in an ink jet printing process.
Accordingly an object of the present invention is to provide recording sheets for use in ink jet recording which have high ink absorptiveness, high ink absorption rate, excellent image quality, excellent water fastness and an excellent light stability.
A further objective is to provide recording materials giving excellent image quality with the wide variety of ink jet printers available on the market today.